Prehospital use of 12-lead computerized electrocardiographs has been proposed to facilitate rapid and accurate treatment for acute myocardial infarction (AMI), but its impact has not yet been evaluated in an interventional randomized clinical trail (RCT). The additional provision on the electrocardiogram(ECG) header of predictions by our acute cardiac ischemia time-insensitive predictive instrument (ACI-TIPI) and thrombolytic predictive instrument (TPI) have been tested in large RCTs in emergency departments (EDs, but not yet in prehospital emergency medical service (EMS) settings. Thus, in this first phase, we will plan the details of an RCT with three arms: 1) EMS standard use of single-lead ECG monitoring during prehospital care and transport: 2) EMS use of 12-lead ECG's with standard printed computerized interpretations: 3) EMS use of 12-lead ECG's that have the predictive instrument predictions in addition to the standard text. Anticipated to be done in US EMS systems representing a diversity of types, settings, and patient populations, and to include 19,500 subjects, the RCT will mesasure impact on specified primary, secondary, and "process" outcomes, and should be able to detect important clinical benefits for individuals with ACI, including AMI, as well as for efficiency of community EMS systems, generalizable nationally.